1. Field of the Invention
The invention is based on a device for measuring at least one parameter of a medium flowing in a line, and more particularly to a device for measuring the air intake mass flow of an internal combustion engine.
2. Description of the Prior Art
DE 197 35 891 A1 has disclosed a measuring body that can be inserted into a flow of an intake line of an internal combustion engine to measure the mass of intake air, which device has flow conduit and a measurement conduit, which is essentially inclined in relation to a longitudinal axis of a line and is part of an adjoining S-shaped deflection conduit. A measuring element is disposed in the measurement conduit. The measuring element can be embodied as a micromechanical sensor part with a dielectric membrane, as has been disclosed, for example, by DE 43 38 891 A1 and U.S. Pat. No. 5,452,610. The penetration of water into the intake line, for example because the road is wet with rain, can contaminate the measuring element. Natural amounts of dissolved salts contained in this water vapor then cause a drift in the characteristic curve due to the buildup of salt encrustation on the membrane of the sensor part. A protected region is in fact produced by the inclination of the measuring body, but dirt particles or fluid particles get into the measurement conduit anyway.
DE 197 35 664 A1 has already disclosed a device in which the measuring element is disposed inside a tubular body that the medium flows through in which an upstream end of the tubular body extends into a filter chamber and has inlet openings there on a circumference surface in order to reduce the action of dirt particles or water droplets on the measuring element. Particularly with severely contaminated air and a high water content in the intake air of the internal combustion engine, there is the danger that the air filter will become laden with water which then penetrates the filter mat and thereby carries dirt particles along with it. On the downstream side of the air filter, the actually clean side, there is now the danger that the intake air will once again carry along dirt particles and water droplets from the filter surface which will then be undesirably deposited on the measuring element and lead to incorrect measurements or to a failure of the measuring element. Through the placement of inlet openings on the circumference surface, the tubular body according to the prior art does in fact reduce the danger of deposits on the measuring element, but this long an embodiment of the tubular body causes an undesirable pressure drop which leads to a reduction of the measurement sensitivity. Moreover, the reduction of the exposure of the measuring element to fluid/solid particles is insufficient to meet the requirements for a fluid input of 20 liter/hour.
The suggestion has also been made to use a deflecting screen in a line in order to separate out fluid particles from flowing air or from a flowing gas. A deflecting screen of this kind, which is connected upstream of an inner tube or in the line, influences the air/water mixture flowing toward the measuring element in such a way that the fluid particles are conveyed against a tube wall or a line wall, while the air remains in a center of the inner tube. However, this phase separation then produces a very large non-stationary dead water region directly downstream of the deflecting screen, which fills up with water during the operation and then permits this water to flow uncontrollably in the direction of the measuring body. In addition, eddies of air from the dead water region also travel randomly to the measuring body and thus interfere with the reproducibility of the sensor signal.
DE 196 52 753 A1 has disclosed a device with a measuring element which contains a flow rectifier and a screen for stabilizing a measurement signal. However, no additional screen or element is used to protect the measuring element from fluids or solid particles.
DE 196 47 081 A1 and U.S. Pat. No. 5,918,279 have disclosed a screen which in different areas, has screen openings with different cross-sections. However, no steps are taken there to prevent a contamination of the measuring element with water and/or solid particles.
The device according to the invention has the advantage over the prior art that an improvement of the measurement results is achieved in a simple manner by virtue of the fact that fluids and/or solid particles are deflected around a measuring element by a protective screen.
It is particularly advantageous to reduce the production of a dead water region and an eddy zone through means disposed downstream of the protective screen or in the protective screen.
One advantageous embodiment for reducing a dead water region is comprised of longitudinal ribs which extend axially in the direction of the dead water region and become thicker in the main flow direction. This produces a greater wall surface area and friction so that a flow speed in the dead water region is reduced and consequently, the dead water region is significantly reduced in size. This leads to a behavior that is constant over time and to reduced signal noise of a measuring element.
It is also advantageous to aspirate the water contained in the dead water region away from this region. This takes place by means of at least one suction opening let into the tubular body in the vicinity of the dead water region. Projections produce an acceleration of the flow in the vicinity of the suction openings.
It is advantageous to embody the protective screen as a turbulence generator in order to reduce an accumulation of water in the dead water region because the flow is forced to spin, which pushes the water more forcefully against an inner wall of the line or the tubular body.
For an integral embodiment, it is advantageous to accommodate the protective screen as a turbulence generator in a recess of the measuring body; there can also be longitudinal ribs and suction openings in a measurement conduit of the measuring body.
It is also advantageous to use a tubular body in a line which contains a measuring body since the tubular body already reduces the contamination by solid and fluid particles.
It is also advantageous to insert the protective screen into the tubular body, which leads to a further significant reduction of the contamination by solid and fluid particles because the flowing medium is deflected.
One possibility for deflecting dirt particles and fluid droplets in the desired direction is advantageously produced when the protective screen extends inclined in relation to flow direction.
In order to reduce the number of parts and the cost of manufacturing, it is advantageous to integrate the protective screen into a flow rectifier disposed upstream of the measuring body.